Hydrogen fuel cell (FC) energy and photovoltaic energy are examples of the front-runners among alternate-energy solutions to address and alleviate the imminent and critical problems of existing fossil-fuel-energy systems: environmental pollution due to high emission level and rapid depletion of fossil fuel. Power conditioning systems (PCS) are required to condition the energy generated by the alternative-energy sources to, forms which can be used readily by the consumers. The choice of PCS topologies for FC energy systems, for example, can be broadly categorized as push-pull and full-bridge based topologies. Push-pull based topology, owing to its low part count, is a good candidate for a low-cost FC converter. However, at higher power it suffers from problems due to transformer flux imbalance and core-saturation.
Because of the symmetrical transformer flux and equal electrical stress distribution, several variations of full-bridge inverter topologies have been found to be useful from the cost and efficiency point of view. However, for high-voltage applications, the voltage stress on the switches increases significantly, thereby degrading the reliability of the overall PCS.
An important variable in the design of the PCS for alternative-energy sources such as fuel-cells is the amount of ripple current the fuel cell can withstand. It is known that a large ripple current will reduce the maximum power output available from the FC stack, but apart from this little is known about the dynamic electrical performance, particularly with regard to long-term effects. Also, since the reactant utilization is known to impact the mechanical nature of a fuel-cell, it is suggested that the varying reactant conditions surrounding the cell (due to ripple current) govern, at least in part, the lifetime of the cells. Both the magnitude and frequency of the ripple current is important. Fuel-cell power electronics for residential and commercial applications are typically designed to have a single or a two-phase output. Single and two-phase inverter systems draw a sinusoidal current component at twice the fundamental frequency. For fuel-cells powering single phase loads (60 Hz), the ripple current of concern is twice the output frequency, i.e., 120 Hz. A limit of 0.15 per-unit (i.e. 15% of its rated current) from 10% to 100% load is specified. Apart from the direct effect on the fuel cell durability, the harmonics have the effect of increasing the power-conditioner copper, iron and dielectric losses and thus increasing the thermal stress on the power stage components. Power derating of the individual components and over sizing is a preventive measure which results in compromise in power density and increased costs.